Active gate control (AGC) which actively controls a gate voltage or a gate current is used as a technique for eliminating a trade-off between a surge voltage and a switching loss during a switching operation in a power switching element that constitutes a semiconductor power conversion device such as an inverter or a converter.
For example, when an insulated gate bipolar transistor (IGBT) is used as a power switching element, a change with time dV/dt of a collector-emitter voltage (hereinbelow, referred to as a collector voltage Vce) during turning-off is fed back to control a discharging speed of a gate charge of the IGBT. Specifically, Patent Literature 1 proposes a technique for switching the discharging speed of the gate charge in the middle of discharge by switching the amount of a gate current injected to the gate of the IGBT. However, the technique of Patent Literature 1 requires gate-off circuits corresponding to the number of switching stages. Thus, a circuit scale, that is, the layout area becomes large.
On the other hand, Patent Literature 2 proposes a circuit that enables the layout area to be reduced in a semiconductor device that has a wide output current, that is, capable of widely switching an output current. Specifically, the semiconductor device generates the output current by a current mirror of a reference current. Further, a wide dynamic range of the output current is achieved by a multistage current mirror.
However, the technique of Patent Literature 2 performs current supply to the gate for turning on an output transistor with the reference current. Thus, as the number of stages of output transistors that constitute the current mirror increases, the reference current varies. That is, every time the output transistor is turned on, the output current varies.
Further, in the circuit configuration as employed in Patent Literature 2, it is commonly desired to make the reference current as small as possible. However, a charge injection speed to the gate of the output transistor constituting the current mirror depends on the reference current. Thus, when the reference current is small, a time required to turn on the output transistor increases. That is, when the technique of Patent Literature 2 is applied to an off circuit of a power switching element, it may not be possible to ensure a response speed from when an instruction of turning off the power switching element is received until when an output transistor is turned on.